1. Field of the Invention
The present invention relates to a pattern inspection apparatus and a pattern inspection method, and more particularly to an apparatus and method with a function of correcting a distortion of a captured image.
2. Description of Related Art
The lithography technology which promotes micro-miniaturization of semiconductor devices is extremely important as being the only process whereby patterns are formed, in the semiconductor manufacturing. In recent years, with the high integration of LSI, the line width (critical dimension) required for semiconductor device circuits is decreasing year by year. Then, in order to form a desired circuit pattern on such semiconductor devices, there is a need for a highly accurate master or “original” pattern (also called a mask or a reticle).
Since the LSI manufacturing requires a tremendous amount of manufacturing cost, it is crucial to improve its yield. One of major factors that decrease the yield of the LSI manufacturing is a pattern defect of a mask used when exposing (transferring) a fine pattern onto a semiconductor wafer by the photolithography technology. In recent years, with miniaturization of an LSI pattern formed on a semiconductor wafer, dimensions of defects to be detected have become extremely small. Thus, a pattern inspection apparatus for inspecting defects of a mask for exposure used in manufacturing LSI needs to be highly accurate.
As an inspection method, it is known that an optical image of a pattern formed on a target object or “sample”, such as a lithography mask, imaged at a predetermined magnification using a magnifying optical system is compared with design data or an optical image of an identical pattern on the target object. For example, the following is known as pattern inspection methods: “die to die inspection” method that compares data of optical images of identical patterns at different positions on the same mask, and “die to database inspection” method that inputs into the inspection apparatus the writing data (design pattern data) converted from pattern-designed CAD data to a format for input to the writing apparatus when writing a pattern on a mask, generates design image data (reference image) based on the input writing data, and compares the generated design image data with an optical image (measurement data) obtained by capturing an image of the pattern. According to the inspection method of the inspection apparatus, a target object is positioned on a stage so that a light flux may scan the object by the movement of the stage. Specifically, the target object is irradiated with a light flux from the light source and the illumination optical system. Light transmitted through the target object or reflected therefrom is focused on a sensor through the optical system. An image captured by the sensor is transmitted as measurement data to a comparison circuit. In the comparison circuit, after position alignment of the images, measurement data and reference data are compared in accordance with an appropriate algorithm. If there is no matching between the data, it is judged that a pattern defect exists (refer to, e.g., Japanese Patent Application Laid-open (JP-A) No. 2008-112178).
As a technique for exposing a fine pattern exceeding a wavelength limit, there are a double exposure technique and a double patterning technique, for example. In these techniques, since two masks are used, there is a case that a local positional deviation of a pattern, which is not usually recognized as a defect in the inspection of each mask, may be a defect when superimposing patterns of both the masks. Therefore, the local positional deviation of the pattern could give a large influence on the yield. Thus, in the inspection apparatus, it is necessary to locally detect a distortion at an absolute position. However, if an image captured by the inspection apparatus is distorted, it becomes difficult to highly accurately detect the distortion at the absolute position. As one of the causes of an image distortion, it can be cited that a relative position between the optical system, from the stage to the sensor, and the stage may deviate due to a thermal expansion, deformation, etc. of the pedestal, etc. of the inspection apparatus. There has been no sufficient solution that suppresses the image distortion caused by the positional deviation of the optical system.
As mentioned above, as one of the causes of an image distortion, it can be cited that a relative position between the optical system, from the stage to the sensor, and the stage may deviate due to a thermal expansion, deformation, etc. of the pedestal, etc. of the inspection apparatus. There has been no sufficient solution that suppresses the image distortion caused by the positional deviation of the optical system.